The present invention relates to a braking device for a motor vehicle, of the type which make it possible to prevent the wheels of this vehicle from locking.
More specifically, the present invention relates to a vehicle braking device of the type comprising a closed hydraulic circuit filled with a hydraulic fluid selectively subjected to various pressures at various points in the circuit, this circuit comprising: a low-pressure reservoir; a first pressure receiver associated with one rear wheel of the vehicle and one inlet of which constitutes a first point of the circuit; a second pressure receiver associated with one front wheel of the vehicle and one inlet of which constitutes a second point of the circuit; a first pressure generator controlled mechanically and one outlet of which constitutes a third point of the circuit; a second pressure generator controlled electrically and one outlet of which constitutes a fourth point of the circuit; a hydraulic-fluid accumulator; a pressure sensor capable of providing a measurement of the pressure of the fluid at the third point of the circuit; and valve means.
Devices of this type are well known in the prior art and have been used for many years.
One of the problems encountered in designing such braking systems lies in the difficulty of constantly improving the performance while at the same time reducing the cost, particularly to allow their use to be as widespread as possible and thus provide optimum safety on the greatest possible number of vehicles.
The present-invention falls within this context and is aimed in particular at allowing an appreciable reduction in the cost of manufacture of devices of this type by optimizing their functions.
To this end, the device of the invention, which in other respects is in accordance with the definition given in the above preamble, is essentially characterized in that the second and fourth points of the circuit are connected to each other so as to be permanently subjected to the same pressure, in that the valve means comprise a first solenoid valve operated to move from an open state in which it connects the second and third points of the circuit to each other, into a closed state in which it isolates the second and third points of the circuit from each other in response to the pressure measurement crossing a predetermined threshold, and in that the pressure delivered by the second pressure generator can be altered electrically.
As a preference, the accumulator has an inlet connected directly to the third point of the circuit and this accumulator is preset to allow hydraulic fluid in only when the fluid pressure at the third point of the circuit is at least equal to three quarters of the said predetermined threshold value.
In a braking device in which a first absorption law connects each variation in volume of fluid let into the accumulator with a corresponding variation in the pressure of the fluid at the third point of the circuit, in which a second absorption law connects each variation in volume of fluid let into the second pressure receiver with a corresponding variation in the fluid pressure at the fourth point of the circuit, and in which the first and second absorption laws have first and second respective gradients, these first and second gradients are advantageously in a ratio of between 0.75 and 1.33.
To further optimize the braking device of the invention, it is preferable to ensure that the valve means further comprise a second solenoid valve selectively connecting the first and third points of the circuit, and a third solenoid valve selectively connecting the first point of the circuit to the low-pressure reservoir, the second and third solenoid valves allowing the pressure in the first pressure receiver to be altered selectively.
In the conventional way, the first pressure generator may comprise a master cylinder which may or may not be associated with a vacuum booster.
The second pressure generator, for its part, may comprise a piston mounted so that it can slide in leaktight fashion in a cylinder and be selectively moved by an electric motor.
By virtue of these features, the device of the invention can combine the advantages of variable boosting on each front wheel of the vehicle with the advantages of anti-lock regulation on each rear wheel, without having to resort to the use of a pump.
It is thus possible, in particular:
to respond to emergency braking situations with variable boosting on each front wheel;
to maintain and/or improve the response times of the braking device by virtue of the direct connection between the master cylinder and each pressure receiver in the initial phase of a braking sequence;
to retain the conventional pedal feel, the conventional dead travel and the conventional triggering threshold of a standard braking device, by fitting the device of the invention with a small-size vacuum booster;
to control manufacturing costs by simplifying the anti-lock function for the wheels on the rear axle of the vehicle;
and to obtain improved safety by using two sources of energy, namely muscular energy and electrical energy and by duplicating the pressure generators that control the front pressure receivers.
Other features and advantages of the invention will emerge clearly from the description thereof given hereafter by way of non-limiting indication with reference to the appended drawings.